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  #1  
Old 10-05-2010, 09:29 PM
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nothingbutford nothingbutford is offline
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How to test camshaft position sensor??

Hi, been a along time since I've been on here. I searched and didn't exactly find what I wanted.

My friend's 98 ranger 2.5 auto has severely limited power and runs very rough. The engine light is on with code P0340 which points to the CMP sensor circuit. I have tested the blue/yellow wire for continuity between the sensor and ecu pin 85 and it seems just fine. Resistance across the CMP sensor is 334ohms......is this normal or even a good test of this sensor? Since the signal wire is fine is it definitely the sensor?

My friend has very limited funds, is it ok to buy a parts store part or is it a must to stick with OEM on this?

Thanks alot!
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1991 F-350 Lariet, CC, LB, SRW, 7.3 Banks turbo, 3.5" exhaust, E4OD, 4.10LS rear.
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Old 10-05-2010, 11:35 PM
g_k50 g_k50 is offline
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Ok, let's go over the nomeclature first. The connector on the CMP sensor has a CMP connection and a SIG RTN connection. With the rounded part of the connector to the left the CMP is PCM Pin 85 is on the top, SIG RTN PCM pin 76 bottom.
Test Steps:
Check for open cmp, sig rtn/cmp gnd and pwr gnd circuits between pcm and cmp sensor.
This is the continuity check? Should be less than 5.0 Ohms, Yes

Then:
Check CMP circuit for short to power in harness by:
Reconnect CMP sensor
Key on, engine off
Measure voltage between CMP and CMP GND/SIG RTN circuits at the PCM connector.
Is voltage less than 1.0 volt? If no repair short circuit in harness

If yes then:
Check CMP circuit for short to GND and SIG RTN in harness by:
Diconnect CMP sensor
Measure resistance between CMP and CMP GND/SIG RTN at the PCM harness connector
Is the resistance greater than 10,000 Ohms? If no, repair shor circuit

If yes then Check for shorts in PCM by:
Reconnect PCM
Measure resistance between CMP and CMP GND/SIG RTN circuits at the CMP harness connector.
Is the resistance greater than 500 Ohms? If no, Replace PCM

If yes then Check CMP sensor output by
Reconnect CMP sensor
Digital Multimeter on AC scale for less than 5.0 volts
Measure voltage between CMP and CMP GND.SIG RTN circuits while running engine at varying rpm.
Does the AC voltage varey greater than 0.1 volt AC? If no Replace CMP sensor.
If yes replace PCM.

No promises made concerning this proceedure which is from a 2001 Powertrain and Emissions Manual.
Your 1998 might be significantly different, but the troubleshooting should be similar.
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Old 10-06-2010, 12:23 AM
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Obviously I don't have the shop manual since it's not my truck, but here is what AutoZone troubleshooting says:

Two Wire Sensors:

See Figure 4

1. With the ignition OFF , install a break-out box between the CMP sensor and PCM.

2. Using a Digital Volt-Ohmmeter (DVOM) set to the voltage function (scale set to monitor less than 5 volts), measure the voltage between break-out box terminals 24 and 46 with the engine running at varying RPM. If the voltage reading varies more than 0.1 volt AC, the sensor is okay.


Not sure if it's correct because the replacement instructions say the A/C and P/S must be removed but I was able to easily reach the sensor from under the truck.
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Old 10-06-2010, 12:43 AM
g_k50 g_k50 is offline
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Sounds like it's a similar troubleshooting step to what I have for the 2001 model. The break-out box is just a way of being able to connect the DVOM or multimeter to the sensor.

What voltage are you reading?

It might be that the camshaft position sensor was reinstalled slightly off, like one gear tooth.
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Old 10-06-2010, 01:32 AM
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Quote:
Originally Posted by g_k50 View Post
Sounds like it's a similar troubleshooting step to what I have for the 2001 model. The break-out box is just a way of being able to connect the DVOM or multimeter to the sensor.

What voltage are you reading?

It might be that the camshaft position sensor was reinstalled slightly off, like one gear tooth.
I think you're thinking about the ones that have a shaft similar to a traditional distributor. My wife's old 2002 taurus had that type and I had to replace it.

That's not how this one works, this one is a hall-affect sensor picking up off the rear of the crank pulley. It can only be bolted on in one position. Most crank sensors work the same way. This sensor was no adjustment for lateral, vertical, or depth movement.

I haven't tested the voltage yet but will tomorrow. I was able to remove it, clean it up really well and reinstall it tonight. Drove around for a bit and the engine light remains off but the engine still lacks power and stumbles randomly.
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Old 10-06-2010, 07:44 PM
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I think I did this right:

Probed pins 85 and 91 at the ecu harness with engine running.

Voltage varied between 0.1-0.5 millivolts. According to the Autozone steps posted above the voltage should vary by MORE than 0.1 volts. I think this confirms a bad sensor.

Interestingly enough, after cleaning and replacing the sensor the engine light hasn't come back on. Engine still runs a bit rough but that's possibly due to the coil secondaries measuring 15k ohms.
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Old 10-06-2010, 07:44 PM
 
 
 
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